Researchers studying clear lakes in Canada have found that climate warming, acid rain and increased ultraviolet sunlight (from depletion of the stratospheric ozone layer) all interact to allow more ultraviolet light to penetrate deeper into lakes, damaging their ecology. (David W. Schindler, P. Jefferson Curtis, Brian R. Parker, and Michael P. Stainton, "Consequences of climate warming and lake acidification for UV-B penetration of North American boreal lakes", Nature Vol. 239, February 22, 1996.) This is the first study to show that the effects of these three factors are related.
These researchers have been studying a group of lakes in north-western Ontario, from 1971 to 1990. These lakes are in the "boreal forest" belt which encircles the northern hemisphere below the arctic tundra and above the steppes, plains and hardwood forests.
Boreal lakes are the most numerous kind of lake on earth. There are perhaps 2 million such lakes. The biggest lakes, Baikal in Russia, and Superior in the US, are boreal lakes.
During the 20-year study period, climate near the Ontario lakes warmed by an average of 1.6 Celsius; the cause of this warming is unknown (though of course global warming must be suspected). The climate warming has had significant impacts on water quality in the lakes.
During the same 20-year period, ultraviolet-B (UV-B) sunlight striking the surface of the lakes increased about 10% as a result of thinning of the stratospheric ozone layer.
During the same period, the lakes became more acidic as a result of acid deposition by rain, snow, and fog.
Dramatic changes have been shown to occur in lake ecosystems exposed to the same intensities of UV-B as those found in eastern Canada today. The photosynthesis of floating plants (phytoplankton) is reduced by increasing UV-B. The deeper the UV-B penetrates into a lake, the more photosynthesis is reduced.
DOC — dissolved organic carbon — is the main factor that prevents the penetration of visible and ultraviolet sunlight in lakes of the boreal and other northern forested regions.
The penetration of UV-B in boreal lakes is known to increase as the concentration of dissolved organic carbon (DOC) decreases. In most boreal lakes, DOC provides an effective shield for aquatic organisms against UV-B, restricting penetration of UV-B to a few tenths of a metre.
The increased penetration of UV-B radiation and resulting harm to micro-organisms can thus create a positive feedback, accelerating the removal of DOC. UV-B reduces DOC, which in turn allows UV-B to penetrate deeper into the water, thus further reducing DOC.
The present study reports that two additional factors have reduced DOC in boreal lakes during the 20-year study period: climate warming and acidification have caused declines in DOC, allowing increased penetration of UV-B. Thus these three factors have worked together to increase the penetration of UV-B deeper into clear lakes.
Although it is not known whether the observed 20-year warming trend is part of a natural cycle or is caused by increasing emissions of greenhouse gases (chiefly carbon dioxide from combustion of coal and oil), this study provides a useful glimpse of the effects of climate warming on boreal lakes.
In addition to the observed increase in air temperature during the 20-year period, precipitation decreased by 25%. Evapotranspiration (total evaporation from earth to atmosphere from lakes, rivers, soil and plants) increased by 35%, and once-permanent streams became intermittent. As a result of reduced precipitation, fewer organic chemicals were carried into the lakes from the land; the net effect of reduced precipitation has been to reduce DOC and clarify the lakes further.
During the 20 years, DOC in lakes in north-western Ontario decreased by an average of 15-20%, causing increases of 22-63% in the average depth of UV-B penetration. As a result, the percentage of the lakes' volume exposed to any given intensity of UV-B increased by 60% for lake 239 and about 22% for lakes 224 and 240.
Acid rain, snow and fog
Acid deposition, from human-caused emissions of oxides of sulphur and nitrogen, is probably the greatest threat to small boreal lakes in Canada and Eurasia. Acidifying sulphur oxide emissions have been reduced by over 50% in Canada. Legislation has been passed to compel similar reductions in the United States by early in the 21st century. However, these measures are estimated to have reduced the potential effect of acid precipitation on Canadian lakes by only about half.
The rapid decline in DOC caused by acidification during the period 1971-1990 caused much greater increases in UV-B penetration than climate warming alone. In the most acidified lake, 302S, the depth of 1% UV-B penetration increased from about 0.3 metres to over 2.8 metres. As a result, the proportion of the lake's volume exposed to greater than 1% of UV-B increased from 6% to nearly 50%.
Overall, the study's authors estimate about 140,000 of the nearly 700,000 lakes in eastern Canada may have DOC concentrations low enough for UV-B penetration to be of concern.
The highest concern, they say, must be for clear, shallow lakes, streams and ponds, where even modest declines in DOC may eliminate the small regions that are deep enough to provide refuges from damaging UV-B radiation.
The authors say it seems possible that some of the many changes to aquatic communities that have been attributed to lake acidification may have involved harmful UV-B exposure.
These results indicate that in aquatic systems, climate warming and/or acidification can increase the exposure of organisms to UV-B much more than changes in UV-B caused by depletion of the stratospheric ozone layer.
The authors say that the decreases in DOC caused by climate warming, drought and acidification should be of much more concern than depletion of stratospheric ozone, so far as UV-B exposure is concerned.
Anything that clarifies the water in an already-clear lake, thus allowing ultraviolet to penetrate deeper than normal beneath the surface, can change the ecosystem in the lake by reducing photosynthesis. Photosynthesis lies at the base of all the planet's food chains.
At a time when a great majority of the world's meteorologists have recently said for the first time that they believe global warming is occurring because of human-induced additions of "greenhouse gases" in the atmosphere, this study provides a preview of some of the effects we might expect from global warming in northern climes.
The study also underscores the point that human-created problems interact in ways that can take decades to recognise, and longer than that to understand. For this reason, it only makes sense for humans to make changes to their environment slowly, which has not been the case for the last 200 years.
[From Rachel's Environment & Health Weekly.]